Aluminum alloy plastic working material and production method therefor

ABSTRACT

The present invention provides an aluminum alloy plastic working material which has a low Young&#39;s modulus, but has an excellent proof stress and a method for efficiently producing the same. The aluminum alloy plastic working material of the present invention comprises: 5.0 to 10.0 wt % of Ca, and the remainder aluminum and unavoidable impurities, a volume ratio of an Al4Ca phase, which is a dispersed phase, is 25% or more. The Al4Ca phase comprises a tetragonal Al4Ca phase and a monoclinic Al4Ca phase, and an intensity ratio (I1/I2) of the highest diffraction peak (I1) attributed to the tetragonal system to the highest diffraction peak (I2) attributed to the monoclinic system, which are obtained by an X-ray diffraction measurement, is 1 or less.

FIELD OF THE INVENTION

The present invention relates to an aluminum alloy plastic workingmaterial which has a low Young's modulus, but has an excellent proofstress, and relates to a method for producing the working material.

Since aluminum has many excellent characteristics such as corrosionresistance, electric conductivity, thermal conductivity, light weight,brightness and machinability, aluminum is used for various purposes. Inaddition, since plastic deformation resistance is small, various shapescan be imparted, and aluminum is also widely used for members subjectedto plastic working such as bending processing.

Here, when the rigidity of the aluminum alloy is high, there is aproblem that the spring back amount increases when the plastic workingsuch as bending processing is performed, and thus it is difficult toobtain dimensional accuracy. Under such circumstances, an aluminum alloymaterial having a low Young's modulus is desired, and a method forlowering the Young's modulus of the aluminum alloy material has beenstudied.

For example, Patent Literature 1 (JP 2011-105982 A) proposes an aluminumalloy containing an Al phase and an Al₄Ca phase, wherein the Al₄Ca phasecontains an Al₄Ca crystallized product, and an average value of thelonger side of the Al₄Ca crystallized product is 50 μm or less.

In the aluminum alloy disclosed in the above Patent Literature 1, themovement of the Al₄Ca crystallized product accompanying the dislocationin the matrix becomes easy, so that the rolling workability of thealuminum alloy can be remarkably improved.

CITATION LIST Patent Literature

Patent Literature 1: JP 2011-105982 A

SUMMARY OF INVENTION Technical Problem

However, as represented by, for example, terminals of electricalequipment, the requirement for the dimensional accuracy of the productwhere aluminum alloys are used has been strict year by year, so thataluminum alloys with lower rigidity are required while maintaining proofstress. Under such technical background, the current situation is thatthe aluminum alloy of Patent Literature 1 cannot sufficiently satisfythe above requirements.

Considering the above problems in the prior arts, an object of thepresent invention is to provide an aluminum alloy plastic workingmaterial which has a low Young's modulus, but has an excellent proofstress, and relates to a method for efficiently producing the workingmaterial.

Solution to Problem

As a result of extensive study with respect to the aluminum alloyplastic working material and production method therefor in order toachieve the above object, the present inventors have found that it isextremely effective that an Al₄Ca phase is used as the dispersed phaseand the crystal structure of the Al₄Ca phase is appropriatelycontrolled, and have reached the present invention.

Namely, the present invention is to provide an aluminum alloy plasticworking material, which comprises:

5.0 to 10.0 wt % of Ca, and

the remainder aluminum and unavoidable impurities,

a volume ratio of an Al₄Ca phase, which is a dispersed phase, is 25% ormore,

the Al₄Ca phase comprises a tetragonal Al₄Ca phase and a monoclinicAl₄Ca phase, and

an intensity ratio (I₁/I₂) of the highest diffraction peak (I₁)attributed to the tetragonal system to the highest diffraction peak (I₂)attributed to the monoclinic system, which are obtained by an X-raydiffraction measurement, is 1 or less.

By addition of Ca, a compound of Al₄Ca is prepared, which has anactivity to lower the Young's modulus of the aluminum alloy. The effectbecomes remarkable when the content of Ca is 5.0% or more. To thecontrary, when added in excess of 10.0%, the casting property decreases,and since particularly casting by continuous casting such as DC castingbecomes difficult, it is necessary to produce by a method with a highproduction cost such as powder metallurgy method. In the case ofproducing by the powder metallurgy method, there is a risk that oxidesformed on the surface of the alloy powder may get mixed in the product,which may lower the proof stress.

In the aluminum alloy plastic working product of the present invention,though the crystal structure of the Al₄Ca phase which is used as thedispersed phase is basically a tetragonal crystal, the present inventorshave intensively studied and found that when the crystal structure ofthe Al₄Ca phase contains a monoclinic crystal, the proof stress do notdecrease so much, but the Young's modulus is greatly decreased. Here,when the intensity ratio (I₁/I₂) of the highest diffraction peak (I₁)attributed to the tetragonal system to the highest diffraction peak (I₂attributed to the monoclinic system, which are obtained by an X-raydiffraction measurement, is 1 or less, the Young's modulus can begreatly lowered while maintaining the proof stress.

Further, it is preferable that the aluminum alloy plastically workingmaterial of the present invention further contains at least one or moreof Fe: 0.05 to 1.0 wt % and Ti: 0.005 to 0.05 wt %.

When Fe is contained in the aluminum alloy, the casting property can beimproved by broadening the solidification temperature range(solid-liquid coexisting region), and thus the casting surface of theingot can also be improved. Further there is an effect that thedispersed crystallized product of Fe makes the eutectic structureuniform. The effect becomes remarkable when the Fe content is 0.05 wt %or more. To the contrary, when contained in excess of 1.0 wt %, theeutectic structure becomes coarse and there is a risk to lower the proofstress.

Ti acts as a refining material of the casted structure and exhibits anaction to improve casting property, extrudability, and rolling property.The effect is remarkable when the Ti content is 0.005 wt % or more. Tothe contrary, even when added in excess of 0.05 wt %, it cannot beexpected to increase the effect of refining the casted structure, and onthe contrary, there is a risk that a coarse intermetallic compound whichis to be the starting point of fracture may be generated. It ispreferable that Ti is added by a rod hardener (Al—Ti—B alloy) during thecasting. B added at this time together with Ti as the rod hardener isacceptable.

Further, in the aluminum alloy plastic working product of the presentinvention, it is preferable that an average crystal grain size of theAl₄Ca phase is 1.5 μm or less. When the average grain size of the Al₄Caphase becomes too large, the proof stress of the aluminum alloydecreases, but when the average grain size is 1.5 μm or less, it ispossible to suppress the decrease of the proof stress.

Further, the present invention provides a method for producing analuminum alloy plastic working material, comprising:

a first step for obtaining a plastic workpiece of an aluminum alloy bysubjecting an aluminum alloy ingot which contains 5.0 to 10.0 wt % of Cawith the remainder aluminum and inevitable impurities, and has a volumeratio of an Al₄Ca phase which is a dispersed phase of 25% or more to aplastic processing, and

a second step for subjecting to a heat treatment in a temperature rangeof 100 to 300° C.

After the first step for obtaining a plastic workpiece of an aluminumalloy by subjecting an aluminum alloy ingot which contains 5.0 to 10.0wt % of Ca with the remainder aluminum and inevitable impurities, andhas a volume ratio of an Al₄Ca phase which is a dispersed phase of 25%or more to a plastic processing, by conducting the step for subjectingto a heat treatment in a temperature range of 100 to 300° C. (Secondstep), a part of the tetragonal Al₄Ca phase can be changed intomonoclinic crystal.

When the holding temperature in the second step is less than 100° C., achange from a tetragonal to a monoclinic crystal is difficult to occur,and when the holding temperature is 300° C. or more, recrystallizationof the aluminum base material occurs and there is a risk that the proofstress will be lowered. The more preferable temperature range of theheat treatment is 160 to 240° C. Though the appropriate time for theheat treatment varies depending on the size and shape of the aluminumalloy material, it is preferable that the temperature of the aluminumalloy material itself is kept at least at the holding temperature for 1hour or more.

In the method for producing the aluminum alloy plastic working materialof the present invention, it is preferable that the aluminum alloy ingotcontains at least one or more of Fe: 0.05 to 1.0 wt % and Ti: 0.005 to0.05 wt %.

When Fe is contained in the aluminum alloy, the casting property can beimproved by broadening the solidification temperature range(solid-liquid coexisting region), and thus the casting surface of theingot can also be improved. Further there is an effect that thedispersed crystallized product of Fe makes the eutectic structureuniform. The effect becomes remarkable when the Fe content is 0.05 wt %or more. To the contrary, when contained in excess of 1.0 wt %, theeutectic structure becomes coarse and there is a risk to lower the proofstress.

Ti acts as a refining material of the casted structure and exhibits anaction to improve casting property, extrudability, and rolling property.The effect is remarkable when the Ti content is 0.005 wt % or more. Tothe contrary, even when added in excess of 0.05 wt %, it cannot beexpected to increase the effect of refining the casted structure, and onthe contrary, there is a risk that a coarse intermetallic compound whichis to be the starting point of fracture may be generated. It ispreferable that Ti is added by a rod hardener (Al—Ti—B alloy) during thecasting. B added at this time together with Ti as the rod hardener isacceptable.

Furthermore, in the method for producing an aluminum alloy plasticworking material according to the present invention, it is preferablethat, before the first step, the aluminum alloy ingot is not subjectedto a heat treatment where the ingot is maintained at a temperature of400° C. or more.

Generally, in the case of preparing an aluminum alloy, before the ingotis subjected to plastic working, a homogenization treatment is carriedout where the ingot is held at a temperature of 400 to 600° C., but whenthis homogenization treatment is performed, the Al₄Ca phase contained inthe aluminum alloy tends to be large, and the average grain size becomeslarger than 1.5 μm. Since the proof stress reduces due to the increasein the average grain size, it is preferable that the homogenizationtreatment at a holding temperature of 400° C. or higher would not beperformed.

Effects of the Invention

According to the present invention, it is possible to provide analuminum alloy plastic working material which has both an excellentproof stress and a low Young's modulus, and a method for efficientlyproducing the working material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process chart relating to the method of producing thealuminum alloy plastic working material of the present invention.

FIG. 2 is an X-ray diffraction pattern of the aluminum alloy plasticworking material.

FIG. 3 is a photograph of the structure of the present aluminum alloyplastic working material 3.

FIG. 4 is a photograph of the structure of the comparative aluminumalloy plastic working material 3.

EMBODIMENTS FOR ACHIEVING THE INVENTION

Hereinafter, the aluminum alloy plastic working material and the methodfor producing therefor of the present invention will be described indetail with reference to the drawings, but the present inventions arenot limited to only those.

1. Aluminum Alloy Plastically Working Material (1) Composition

The aluminum alloy plastic working material includes 5.0 to 10.0 wt % ofCa, and the remainder aluminum and unavoidable impurities. In addition,it is preferable to further contain at least one or more of Fe: 0.05 to1.0 wt % and Ti: 0.005 to 0.05 wt %.

Each component element will be explained below.

Ca: 5.0 to 10.0 wt % (Preferably 6.0 to 8.0 wt %)

Ca forms a compound of Al₄Ca and has the activity to lower the Young'smodulus of the aluminum alloy. The effect becomes remarkable when thecontent of Ca is 5.0% or more. To the contrary, when added in excess of10.0%, the casting property decreases, and since particularly casting bycontinuous casting such as DC casting becomes difficult, it is necessaryto produce by a method with a high production cost such as powdermetallurgy method. In the case of producing by the powder metallurgymethod, there is a risk that oxides formed on the surface of the alloypowder may get mixed in the product, which may lower the proof stress.

Fe: 0.05 to 1.0 wt %

When Fe is contained, the casting property can be improved by broadeningthe solidification temperature range (solid-liquid coexisting region),and thus the casting surface of the ingot can also be improved. Furtherthere is an effect that the dispersed crystallized product of Fe makesthe eutectic structure uniform. The effect becomes remarkable when being0.05 wt % or more, and to the contrary, when contained in excess of 1.0wt %, the eutectic structure becomes coarse and there is a risk to lowerthe proof stress.

Ti: 0.005 to 0.05 wt %

Ti acts as a refining material of the casted structure and exhibits anaction to improve casting property, extrudability, and rolling property.The effect is remarkable when being 0.005 wt % or more, and to thecontrary, even when added in excess of 0.05 wt %, it cannot be expectedto increase the effect of refining the casted structure, and on thecontrary, there is a risk that a coarse intermetallic compound which isto be the starting point of fracture may be generated. It is preferablethat Ti is added by a rod hardener (Al—Ti—B alloy) during the casting. Badded at this time together with Ti as the rod hardener is acceptable.

Other Component Elements

As long as the effects of the present invention are not impaired, it ispermissible to contain other elements.

(2) Structure

The aluminum alloy plastic working material has a volume ratio of anAl₄Ca phase, which is a dispersed phase, is 25% or more, the Al₄Ca phasecomprises a tetragonal Al₄Ca phase and a monoclinic Al₄Ca phase, and anintensity ratio (I₁/I₂) of the highest diffraction peak (I₁) attributedto the tetragonal system to the highest diffraction peak (I₂) attributedto the monoclinic system, which are obtained by an X-ray diffractionmeasurement, is 1 or less.

The tetragonal Al₄Ca phase and the monoclinic Al₄Ca phase exist in theAl₄Ca phase, which is a dispersed phase, and the volume ratio of thecombined Al₄Ca phase is 25% or more. By making the volume ratio of theAl₄Ca phase to 25% or more, it is possible to impart an excellent proofstress to the aluminum alloy plastic working material.

Further, it is preferable that an average crystal grain size of theAl₄Ca phase is 1.5 μm or less. When the average grain size of the Al₄Caphase exceeds 1.5 μm, there is a risk that the proof stress of thealuminum alloy plastic working material decreases.

Though the crystal structure of the Al₄Ca phase is generally atetragonal crystal, the present inventors have intensively studied andfound that when the monoclinic crystal structure exists in the Al₄Caphase, the proof stress do not almost decrease, but the Young's modulusis greatly decreased. It is not necessary that all crystal structure ofthe Al₄Ca phases is monoclinic, and it may be in the state of beingmixed with the tetragonal crystal. The existence of the Al₄Ca phasewhich has the monoclinic crystal structure can be identified, forexample, by measuring the diffraction peak with X ray diffractionmethod.

Regarding the Al₄Ca phases, the intensity ratio (I₁/I₂) of the highestdiffraction peak (I₁) attributed to the tetragonal system to the highestdiffraction peak (I₂) attributed to the monoclinic system, can generallybe obtained by a X-ray diffraction measurement. The lattice constants ofthe tetragonal Al₄Ca are a=0.4354 and c=1.118, and the lattice constantsof the orthorhombic Al₄Ca are a=0.6158, b=0.6175, c=1.118, 6=88.9°.

2. Method for Producing Aluminum Alloy Plastic Material

FIG. 1 shows a process chart of the aluminum alloy plastic workingmaterial of the present invention. The method for producing the aluminumalloy plastic working material of the present invention includes a firststep (S01) of subjecting an aluminum alloy ingot to plastic working, anda second step (S02) of applying a heat treatment. Each step and the likewill be explained herein below.

(1) Casting

After subjecting the aluminum alloy molten metal having the compositionof the above-mentioned aluminum alloy plastic working material of thepresent invention to conventionally known molten metal cleaningtreatments such as desulfurization treatment, degassing treatment, andfiltration treatment, the molten metal is casted into an ingot having adesired shape.

There is no particular restriction on the casting method, and variousconventionally known casting methods can be used. For example, it ispreferable, by using a continuous casting method such as DC casting, tocast into a shape that the plastic working (extrusion, rolling, forging,etc.) in the first step (S01) is easy to be performed. In the casting, arod hardener (Al—Ti—B) may be added to improve casting property.

Generally, in the case of preparing an aluminum alloy, before the ingotis subjected to plastic working, a homogenization treatment is carriedout where the ingot is held at a temperature of 400 to 600° C., but whenthis homogenization treatment is performed, the Al₄Ca phase tends to belarge (average grain size of 1.5 μm or larger), and since the proofstress of the aluminum alloy reduces, it is preferable that thehomogenization treatment would not be performed in the method forproducing aluminum alloy plastic working material according to thepresent invention.

(2) First Step (S01)

The first step (S01) is a step of subjecting the aluminum alloy ingotobtained in (1) to the plastic working to obtain a desired shape.

For the plastic working such as extrusion, rolling, or forging, eitherhot working or cold working may be used, or a plurality of them may becombined. By performing the plastic working, the aluminum alloy becomesa processed structure, and the proof stress is improved. In the stage ofthe plastic working, most Al₄Ca phases contained in the aluminum alloyhave the tetragonal crystal structure.

(3) Second Step (S02)

The second step (S02) is a step for applying the heat treatment to thealuminum alloy plastic working material obtained in the first step(S01).

By subjecting the aluminum alloy plastic working material subjected toplastic working in the first step (S01) to the heat treatment at 100 to300° C., a part of the tetragonal Al₄Ca phase can be converted into themonoclinic crystal. The change from the tetragonal to the monoclinic isdifficult to occur when the holding temperature is less than 100° C. Onthe other hand, since, when the holding temperature is 300° C. orhigher, recrystallization of the aluminum base material may occur andthere is a risk that the proof stress may be reduced, the holdingtemperature of the heat treatment is preferably 100 to 300° C., morepreferably 160 to 240° C.

Though the optimum period of time for the heat treatment variesdepending on the size and shape of the aluminum alloy plastic workingmaterial to be treated, it is preferable that the temperature of atleast the aluminum alloy plastic working material is kept at the aboveholding temperature for 1 hour or more.

The representative embodiments of the present invention have beendescribed above, but the present invention is not limited only to theseembodiments, and various design changes are possible, and all suchdesign changes are included in the technical scope of the presentinvention.

EXAMPLES Example

An aluminum alloy having the composition shown Table 1 was cast into aningot (billet) of φ8 inches by a DC casting method without anyhomogenization treatment, and then, plastic-working at an extrusiontemperature of 500° C. to obtain a plate having a width of 180 mm×athickness of 8 mm. Then, after cold rolling to a thickness of 5 mm, aheat treatment was carried out to hold at 200° C. for 4 hours to obtainthe present aluminum alloy working plastic material.

TABLE 1 (unit: wt %) Ca Fe Ti Al Present aluminum alloy plastic workingmaterial 1 5.2 0.001 0.002 Bal. Comparative aluminum alloy plasticworking material 1 Present aluminum alloy plastic working material 2 6.20.05 0.002 Bal. Comparative aluminum alloy plastic working material 2Present aluminum alloy plastic working material 3 7.3 0.05 0.01 Bal.Comparative aluminum alloy plastic working material 3 Present aluminumalloy plastic working material 4 8.1 0.001 0.01 Bal. Comparativealuminum alloy plastic working material 4 Present aluminum alloy plasticworking material 5 9.5 0.05 0.05 Bal. Comparative aluminum alloy plasticworking material 5

The obtained present aluminum alloy plastic working material 3 wassubjected to the X-ray diffraction measurement to measure the positionpf the peak of the Al₄Ca phase. In the X-ray diffraction measurement, aspecimen of 20 mm×20 mm was cut out from the plate-like aluminum alloyplastic working material, the surface layer portion was removed by about500 μm, and then a θ-2θ measurement was carried out with respect to theregion from a Cu—Kα beam source. The results are shown in FIG. 2. Theintensity ratio (I₁/I₂) of the highest diffraction peak (I₁) attributedto the tetragonal system to the highest diffraction peak (I₂) attributedto the monoclinic system was 0.956.

In addition, JIS-14B specimens were cut out from the present aluminumalloy plastic working materials 1 to 5, and the Young's modulus andproof stress were measured by a tensile test. The obtained results areshown in Table 2. In addition, the volume ratio of the dispersed phase(Al₄Ca phase) calculated from the structural observation by the opticalmicroscope are shown in Table 2.

The present aluminum alloy plastic working materials 6 to 9 wereobtained in the same manner as in the case of the present aluminum alloyplastic working material 3 except that the heat treatment temperaturewas any one of 100° C., 160° C., 240° C. and 300° C. In addition, in thesame manner as in the case of the present aluminum alloy plastic workingmaterials 1 to 5, the Young's modulus and proof stress were measured bya tensile test. The obtained results are shown in Table 3.

Comparative Example

An aluminum alloy having the composition shown Table 1 was cast into aningot (billet) of φ8 inches by a DC casting method without anyhomogenization treatment, and then, plastic-working at an extrusiontemperature of 500° C. to obtain a plate having a width of 180 mm×athickness of 8 mm. Thereafter, the cold rolling to a thickness of 5 mmwas carried out to obtain the comparative aluminum alloy plastic workingmaterials 1 to 5.

The obtained comparative aluminum alloy plastic working material 3 wassubjected to the X-ray diffraction measurement to measure the positionpf the peak of the Al₄Ca phase. In the X-ray diffraction measurement, aspecimen of 20 mm×20 mm was cut out from the plate-like aluminum alloyplastic working material, the surface layer portion was removed by about500 μm, and then a θ-2θ measurement was carried out with respect to theregion from a Cu—Kα beam source. The results are shown in FIG. 2. Theintensity ratio (I₁/I₂) of the highest diffraction peak (I₁) attributedto the tetragonal system to the highest diffraction peak (I₂) attributedto the monoclinic system was 1.375.

In addition, JIS-14B specimens were cut out from the comparativealuminum alloy plastic working materials 1 to 5, and the Young's modulusand proof stress were measured by a tensile test. The obtained resultsare shown in Table 2.

The comparative aluminum alloy plastic working materials 6 and 7 wereobtained in the same manner as in the case of the present aluminum alloyplastic working material 3 except that the heat treatment temperaturewas 90° C. and 310° C. In addition, in the same manner as in the case ofthe comparative aluminum alloy plastic working materials 1 to 5, theYoung's modulus and proof stress were measured by a tensile test. Theobtained results are shown in Table 3.

The comparative aluminum alloy plastic working material 8 was obtainedin the same manner as in the case of the present aluminum alloy plasticworking material 3 except that, after casting in an ingot (billet), thehomogenization treatment was carried out while holding at 550° C. Inaddition, JIS-14B specimen was cut out from the comparative aluminumalloy plastic working material 8, and the Young's modulus and proofstress were measured by a tensile test. The obtained results are shownin Table 4. The Young's modulus and the proof stress of the presentaluminum alloy plastic working material 3 which is different only in thepresence or absence of homogenization treatment are also shown ascomparison data.

TABLE 2 Volume ratio of dispersed Young's Proof Tensile Heat phasemodulus stress strength treatment (%) (GPa) (MPa) (MPa) Present aluminumalloy plastic working material 1 Did 26.7 58 147 245 Present aluminumalloy plastic working material 2 31.2 167 258 Present aluminum alloyplastic working material 3 35.9 53 169 254 Present aluminum alloyplastic working material 4 39.9 51 208 272 Present aluminum alloyplastic working material 5 44.3 48 173 269 Comparative aluminum alloyplastic working material 1 Non — 67 176 259 Comparative aluminum alloyplastic working material 2 — 64 184 268 Comparative aluminum alloyplastic working material 3 — 61 189 265 Comparative aluminum alloyplastic working material 4 — 57 222 285 Comparative aluminum alloyplastic working material 5 — 56 186 276

From the results shown in Table 2, when comparing the present aluminumalloy plastic working material having the same composition with thecomparative aluminum alloy plastic working material, the Young's modulusof the aluminum alloy plastic working materials of the present invention(the present aluminum alloy plastic working materials 1 to 5) aregreatly lower than the Young's modulus of the comparative aluminum alloyplastic working materials 1 to 5 which were not subjected to the heattreatment. On the other hand, the proof stress and tensile strength ofthe present aluminum alloy plastic working materials 1 to 5 are notgreatly reduced as compared with the comparative aluminum alloy plasticworking materials 1 to 5. It is clear that the volume ratios of thedispersed phase (Al₄Ca phase) in the aluminum alloy plastic workingmaterials of the present invention are 25% or more.

TABLE 3 Heat treatment Young's Proof Tensile temperature modulus stressstrength (° C.) (GPa) (MPa) (MPa) Present aluminum alloy plastic workingmaterial 6 100 54 187 267 Present aluminum alloy plastic workingmaterial 7 160 54 172 262 Present aluminum alloy plastic workingmaterial 8 240 53 167 252 Present aluminum alloy plastic workingmaterial 9 300 52 161 240 Comparative aluminum alloy plastic workingmaterial 6 90 59 195 275 Comparative aluminum alloy plastic workingmaterial 7 310 53 143 231

From the results shown in Table 3, when the holding temperature of theheat treatment is 90° C. (comparative aluminum alloy plastic workingmaterial 6), the Young's modulus shows a high value (almost notlowered). In addition, when the holding temperature of the heattreatment is 310° C. (comparative aluminum alloy plastic workingmaterial 7), though the Young's modulus is lowered, the proof stress andtensile strength are simultaneously lowered. From the results, when theholding temperature of the heat treatment was 310° C., it is consideredthat the recrystallization of the plastic working structure progressed.

The structural photographs of the present aluminum alloy plastic workingmaterial 3 and the comparative aluminum alloy plastic working material 8by an optical microscope are shown in FIG. 3 and FIG. 4, respectively.In the structure photograph, the black region is the Al₄Ca phase, andthe average crystal grain size of the Al₄Ca phase is measured by imageanalysis. The obtained results are shown in Table 4.

TABLE 4 Average crystal grain size of Young's Proof TensileHomogenization Al₄Ca phase modulus stress strength treatment (μm) (GPa)(MPa) (MPa) Comparative aluminum alloy plastic working material 8 Did1.56 53 158 229 Present aluminum alloy plastic working material 3 Non1.15 53 169 254

From the results shown in Table 4, when subjecting to the homogenizationtreatment maintained at 550° C. (comparative aluminum alloy plasticworking material 8), it is recognized that the proof stress and thetensile strength are reduced. Here, the average crystal grain size ofthe Al₄Ca phase is increased by the homogenization treatment to 1.56 μm.It is considered that the proof stress and the tensile strength arereduced due to the increase in the average crystal grain size.

1. An aluminum alloy plastic working material, which comprises: 5.0 to10.0 wt % of Ca, and the remainder aluminum and unavoidable impurities,a volume ratio of an Al₄Ca phase, which is a dispersed phase, is 25% ormore, the Al₄Ca phase comprises a tetragonal Al₄Ca phase and amonoclinic Al₄Ca phase, and an intensity ratio (I₁/I₂) of the highestdiffraction peak (I₁) attributed to the tetragonal system to the highestdiffraction peak (I₂) attributed to the monoclinic system, which areobtained by an X-ray diffraction measurement, is 1 or less.
 2. Thealuminum alloy plastic working material according to claim 1, furthercomprising at least one or more of Fe: 0.05 to 1.0 wt % and Ti: 0.005 to0.05 wt %.
 3. The aluminum alloy plastic working material according toclaim 1, wherein an average crystal grain size of the Al₄Ca phase is 1.5μm or less.
 4. A method for producing an aluminum alloy plastic workingmaterial, comprising: a first step for obtaining a plastic workpiece ofan aluminum alloy by subjecting an aluminum alloy ingot which contains5.0 to 10.0 wt % of Ca with the remainder aluminum and inevitableimpurities, and has a volume ratio of an Al₄Ca phase which is adispersed phase of 25% or more to a plastic processing, and a secondstep for subjecting to a heat treatment in a temperature range of 100 to300° C.
 5. The method for producing an aluminum alloy plastic workingmaterial according to claim 4, wherein aluminum alloy ingot contains atleast one or more of Fe: 0.05 to 1.0 wt % and Ti: 0.005 to 0.05 wt %. 6.The method for producing an aluminum alloy plastic working materialaccording to claim 4, wherein, before the first step, the aluminum alloyingot is not subjected to a heat treatment where the ingot is maintainedat a temperature of 400° C. or more.
 7. The aluminum alloy plasticworking material according to claim 2, wherein an average crystal grainsize of the Al₄Ca phase is 1.5 μm or less.